How to define a new buffer?

I want to define a new buffer, for example, FMQ (fayalite-magnetite-quartz) with the equilibria of 3Fe2SiO4 + O2 = 2Fe3O4 + 3SiO2. Following the tutorial of User's Guide, it seems like that the following input should be inserted into the database:

FMQ-BUFFER O(2) FMQ *FMQ

ST 0.000 000000.000 000.000000 0.000000

CP1 0.000000 0.00000 0.00 0.0

COM MAGNETITE[2]QUARTZ[3]FAYALITE[-3] 0 MAGNETITE?

But I am not sure which phase should be excluded?

More generally, a buffer may be described as ΔFMQ-1 or ΔFMQ+1. In this case, what should I input in the database?

Everybody is welcomed to share your experience and skill. Thanks a lot.